How to modify parent scope variable using Powershell

The parent scope can actually be modified directly with Set-Variable -Scope 1 without the need for Script or Global scope usage. Example:

$val = 0
function foo {
    Set-Variable -scope 1 -Name "Val" -Value "10"
}
foo
write "The number is: $val"

Returns:

The number is: 10

More information can be found in the Microsoft Docs article About Scopes. The critical excerpt from that doc:

Note: For the cmdlets that use the Scope parameter, you can also refer to scopes by number. The number describes the relative position of one scope to another. Scope 0 represents the current, or local, scope. Scope 1 indicates the immediate parent scope. Scope 2 indicates the parent of the parent scope, and so on. Numbered scopes are useful if you have created many recursive scopes.

---

Be aware that recursive functions require the scope to be adjusted accordingly:

$val = ,0
function foo {
    $b = $val.Count
    Set-Variable -Name 'val' -Value ($val + ,$b) -Scope $b
    if ($b -lt 10) {
        foo
    }
}

You don't need to use the global scope. A variable with the same name could have been already exist in the shell console and you may update it instead. Use the script scope modifier. When using a scope modifier you don't include the $ sign in the variable name.

$script:val=10

Let me point out a third alternative, even though the answer has already been made. If you want to change a variable, don't be afraid to pass it by reference and work with it that way.

$val=1
function bar ($lcl) 
{
    write "In bar(), `$lcl.Value starts as $($lcl.Value)"
    $lcl.Value += 9
    write "In bar(), `$lcl.Value ends as $($lcl.Value)"
}
$val
bar([REF]$val)
$val

That returns:

1
In bar(), $lcl.Value starts as 1
In bar(), $lcl.Value ends as 10
10

If you want to use this then you could do something like this:

$global:val=0 
function foo()
{
    $global:val=10 
}
foo
write "The number is: $val"